Pneumatic rubber tires are conventionally prepared with a rubber tread which are typically composed of a blend of elastomers.
It is often desired for the rubber composite for the tread to be designed to provide an optimum of traction, treadwear and/or rolling resistance properties for the tire, depending somewhat upon the intended use for the tire.
This invention is primarily directed to passenger vehicle tires where it is desired for the tire tread to have good traction on the road, good treadwear as the tire is used in service and relatively low rolling resistance for vehicular fuel economy.
It is well recognized that if it is desired to particularly enhance one of the above three tire tread properties, often one or more of the other two properties are often somewhat reduced or are otherwise often compromised. A suitable balance of such properties is considered herein as being difficult or not readily achievable with a single elastomer in a tire tread so that, therefore, a blend of elastomers is conventionally used for such purpose.
Various diene-based elastomers, including individual polymers of isoprene and butadiene and copolymers thereof either with each other and with vinyl aromatic materials such as, for example, styrene, have been used in various tire tread compositions. For example, cis 1,4-polyisoprene, 3,4-polyisoprene cis 1,4-polybutadiene, styrene/butadiene copolymer and isoprene/butadiene copolymer elastomers have been used in various combinations in tire tread compositions.
Isoprene/butadiene elastomers have been used or suggested for use in various rubber compositions for tire treads. For example, see U.S. Pat. No. 5,294,663. Isoprene/butadiene elastomers can be made with a wide range of glass transition temperatures depending upon the microstructure, such as for example, from about -10.degree. to about -100.degree. C. In general, the more vinyl, or 1,2- units, and/or 3,4-polyisoprene content, in the copolymer the higher its Tg.
Where it is desired to promote a reduction in rolling resistance and improved treadwear for a tire tread, an isoprene/butadiene copolymer elastomer having a very low Tg in a range of, for example, about -70.degree. to about -100.degree. C. is considered herein to be more beneficial than an isoprene/butadiene elastomer with a higher Tg temperature range. However, such copolymer elastomer, as compared to a similar copolymer elastomer having a higher Tg in a range of about -30.degree. to -50.degree. C. is considered to present a compromise in a traction property for the tire tread. Such isoprene/butadiene copolymer elastomer with the substantially lower Tg temperature range may be sometimes referred to herein as a "low Tg IBR".
Such an isoprene/butadiene copolymer elastomer having a Tg in a range of about -75.degree. to about -90.degree. C., has previously been suggested for use in a tire tread composition composed of such elastomer, cis 1,4-polyisoprene natural rubber and, optionally, styrene/butadiene copolymer rubber.
It is considered herein that cis 1,4-polyisoprene natural rubber typically has a Tg of about -65.degree. C. and that a styrene/butadiene copolymer elastomer containing about 5 to about 30 percent styrene would be expected to have a Tg in a range of about -40.degree. to about -70.degree. C. depending primarily on the styrene content.
Therefore, such low Tg IBR, natural rubber, and styrene/butadiene elastomers have somewhat comparable Tg's in an overall range of about -40.degree. to about -90.degree. C. It is considered herein that these three rubbers would be substantially compatible in a sense of a compounded rubber blend exhibiting a single Tg or a relatively broad Tg as compared to a multiplicity of Tg's.
While it is recognized that various tires have heretofore been provided, or proposed, with rubber tread composed of various elastomers, including the aforesaid elastomers, it is desired to provide a tire with a rubber tread of novel elastomer blend.
The glass transition temperatures, or Tg's referred to herein, represent the glass transition temperatures of the respective elastomers in their uncured, uncompounded state. A Tg can be suitably determined by a differential scanning calorimeter at a temperature rate of increase of 1.degree. C. per minute.